docs: add additional instructions for Firefox

apps/extension/FIREFOX_README.md

@@ -21,11 +21,28 @@ several local packages; their sources are in:
 
 ## Build instructions
 
+If you don't already have Node v20 LTS and NPM v10, install now via `nvm`:
+
+```bash
+wget -qO- https://raw.githubusercontent.com/nvm-sh/nvm/v0.40.1/install.sh | bash
+
+# Enable nvm in current shell
+export NVM_DIR="$([ -z "${XDG_CONFIG_HOME-}" ] && printf %s "${HOME}/.nvm" || printf %s "${XDG_CONFIG_HOME}/nvm")"
+[ -s "$NVM_DIR/nvm.sh" ] && \. "$NVM_DIR/nvm.sh"
+
+# Install v20 LTS
+nvm install v20.9.0
+```
+
 These instructions should work for the default reviewer build environment.
 
 ```bash
 sudo apt install protobuf-compiler build-essential
 curl https://sh.rustup.rs -sSf | sh
+
+# Make sure to pull cargo into your current environment:
+source ~/.bashrc
+
 npm install -g yarn
 export PUPPETEER_SKIP_DOWNLOAD=true
 yarn